1. Field of the Invention
The present invention relates to a power unit having a built-in battery and which is suitable for use with small electronic instruments. More particularly, the present invention relates to a power unit to supply electric power to a load and having a built-in battery, wherein the power unit charges the battery even when electric power is being supplied to the load from an external power source at an external power source voltage lower than the battery voltage.
2. Description of the Related Art
Conventional power units used in personal computers and other small electronic instruments have built-in batteries and supply electric power to a load by connecting an AC adapter that converts commercial alternating current power into a specified direct current voltage. When the power unit is not connected to the AC adapter, the power unit supplies electric power to the load from the built-in battery. The conventional power units include a battery charger that uses external electric power to charge the built-in battery when connected to an external power source.
FIGS. 9A-9C illustrate a conventional type of power unit and its operation. More specifically, FIG. 9A illustrates a conventional power circuit; FIG. 9B is a timing diagram illustrating the switching action of a transistor Q1 of the power circuit shown in FIG. 9A; and FIG. 9C is a diagram illustrating the operation of the circuit shown in FIG. 9A.
As shown in FIG. 9A, an AC adapter connecting member 110 is connected to an AC adapter and receives a supply of electric power. A load 111 is supplied electric power from a battery 112 when electric power is not being supplied to the AC adapter connecting member 110 from an external power source. A charge/discharge control circuit 113 controls the charging of the battery 112 by switching the transistor Q1. A charge/discharge monitor circuit 114 monitors whether the battery 112 is charging or discharging, and, if the battery 112 is charging, the charge/discharge monitor circuit 114 monitors the state of the charge and controls switching of the transistor Q1. A battery protection switch 115 connected to the battery 112 shuts off when the battery 112 reaches a discharge final voltage (Vdead) so that the battery 112 does not discharge.
The power circuit includes capacitors C1, C2, an inductor L1, and a diode D1. During the time that the transistor Q1 is off, the diode D1 sends a flywheel electric current to the circuit L1-C1-D1. A diode D2 prevents electric current from flowing from the battery 112 to the AC adapter connecting member 110. A diode D3 is connected across the switching transistor Q1, and is a parasitic diode for the switching transistor Q1.
The switching action of the transistor Q1 controls the voltage applied to the battery 112. FIG. 9B illustrates the switching cycle of the transistor Q1. As shown in FIG. 9B, the switching period of the transistor Q1 is Ts; the on period of the transistor Q1 is Ton; and the off period of the transistor Q1 is Toff. FIG. 9C illustrates a relation between an external power source voltage Vin, a battery voltage Vbat, a discharge final voltage Vdead, a range of voltage Vop that operates the load 111, and a minimum voltage which operates the load Vop (min) (i.e., the minimum operating voltage) for the operation of the circuit shown in FIG. 9A.
The circuit shown in FIG. 9A operates as described below. The battery protection switch 115 remains on while the battery 112 has not yet reached the discharge final voltage Vdead. When the power unit is connected to the AC adapter, the external power source voltage Vin is greater than the battery voltage Vbat, and the power unit reduces the external power source voltage Vin and charges the battery 112. When the AC adapter connecting member 110 is connected to the AC adapter, the electric power input to the AC adapter connecting member 110 is supplied to the load 111 via the diode D2. The charge/discharge monitor circuit 114 monitors the condition of the AC adapter connecting member 110 connection and the status of the charge on the battery 112. The charge/discharge monitor circuit 114 relays the status of the charge on the battery 112 to the charge/discharge control circuit 113. If the battery 112 is fully charged, the charge/discharge control circuit 113 turns off the transistor Q1 and, in general, adjusts the period of time the transistor Q1 is switched on depending on the status of the charge on the battery 112. At this time, the relation between the voltage Vbat of the battery 112, the externally input voltage Vin, the switching time Ts of the transistor Q1, and the on time Ton of transistor Q1 is given by the following equation:
Vbat=Tonxc3x97Vin/Ts.
Therefore, by adjusting the length of time Ton depending on the status of the charge on the battery 112, the reduction of the external power source voltage Vin is adjusted and the charging of the battery 112 can be controlled.
The charge/discharge monitor circuit 114 detects when no electric power is being supplied from an external source because of various causes, such as the AC adapter connecting member 110 not being connected to the AC adapter, and relays the detected information to the charge/discharge control circuit 113. The charge/discharge control circuit 113 turns transistor Q1 on and supplies electric power from the battery 112 to the load 111. At this time, electric current is prevented from flowing from the battery 112 to the AC adapter connecting member 110 by the diode D2, thus preventing unnecessary consumption of the battery 112.
FIG. 9C illustrates a relation between the charge/discharge of the battery 112 and the load voltage both when electric power is being supplied from an external power source and when electric power is not being supplied from an external power source. More particularly, FIG. 9C illustrates a relationship between the externally supplied voltage Vin; the terminal voltage Vbat of the battery 112; the operating voltage of the load Vop, which is the voltage range that operates the load 111; the minimum operating voltage Vop (min) of the load 111; and the discharge final voltage Vdead, which is the minimum battery voltage permitted by the battery 112.
As shown in FIG. 9C, the period AB is the length of time spent charging the battery 112 (Tchg). The period BC is the length of time the battery 112 is fully charged. The time at which the external power source (AC adapter) is disconnected is represented by C. The period AC is the length of time the battery 112 is connected to an external power source. The time at which the battery voltage Vbat reaches the minimum load operating voltage Vop is represented by D. The period CD is the length of battery 112 discharge time (Tdis). The time at which the battery voltage Vbat reaches the discharge final voltage (Vdead) is represented by E.
During the time period AB, the transistor switch Q1 is adjusted and the battery 112 is charged. During the time period BC, the battery 112 is fully charged and transistor Q1 is switched off. At time C, the external power source (AC adapter) is disconnected and transistor switch Q1 is switched on to supply electric power from the battery 112 to the load 111. During the time period CD, the battery 112 discharges. The battery protection switch 115 is turned off and voltage supply to the load 111 is halted when the battery voltage Vbat reaches the minimum operating voltage of the load Vop (min) at time D. At this time however, even when the supply of electric power to the load 111 from the battery 112 is halted at the point in time D, there still remains some power left in the battery 112 before it reaches the discharge final voltage Vdead. Therefore, use of the battery 112 is halted before the battery 112 is completely drained.
In the above-described manner, the conventional power unit with a built-in battery halts discharge from the battery 112 to the load 111 even though some power remains in the battery. For this reason, the battery""s potential has not been fully utilized.
Furthermore, conventional power units always require the external power voltage Vin to be higher than the battery voltage Vbat. Moreover, because the battery voltage Vbat is an unstabilized power source, it can not directly supply loads which require that voltage be supplied at a specified voltage, such as logic circuits. Therefore, in accordance with the conventional power units, a specified-voltage power source, such as a DC/DC converter, has to be connected, adding more parts and leading to increased costs. Also, the conventional power units require a battery protection switch to prevent overdischarge of the battery.
It is an object of the present invention to provide a power unit having a built-in battery that can provide voltage from the battery for a longer period of time by fully utilizing the potential of the battery built into the power unit.
It is another object of the present invention to provide a power unit that can charge a battery even when electric power is being supplied to a load from an external power source at an external power source voltage that is lower than the battery voltage by fully increasing the battery voltage and supplying electric power from the battery to the load at a sufficiently high voltage.
Object and advantages of the present invention are achieved with a power unit that charges a battery power source when supplying electric power to a load from an external power source and supplies electric power to the load from the battery power source when the supply of electric power from the external power source is cut off, wherein the power unit includes a converter to boost the output voltage from the battery power source and to supply the output voltage to the load when the battery power source discharges.
Objects and advantages of the present invention are achieved with a power unit that charges a battery power source when supplying electric power to a load from an external power source and supplies electric power to the load from the battery power source when the supply of electric power from the external power source is cut off, wherein the power unit includes a converter to boost the input voltage from the battery power source and to charge the battery power source when the battery power source is charged.